DVL manual mode

Our application requires very short distance from DVL to flat steel structure. We have mounted DVL about 0.4m above the steel plate and performing some tests in pool. We are getting a lot of reflections and the readings are useless. We have tried the manual mode but there are some issues. We are setting minimum fly height to 0.3m (it is possible to set it even to 0.1m) and we would like to set maximum fly height to 0.5m. Unfortunately minimum value of maximum fly height is 2.0m. Is it possible to limit the distance boundaries so much? What is the meaning of other values set in MANUAL MODE?